Every single awkward question about male body hair, answered

Like, how come eyebrow hairs stop growing but beard hairs don't?

Have you ever noticed how chest hair never grows past a certain point? It grows about an inch long, and then it stops. Trimming it feels pointless if you don’t mind that particular length. Other body parts are a different story. In general, I favor curated male body hair. I’m an advocate of trimmed armpit hair, for instance, since it sometimes grows outside its bounds. Your partner is probably an advocate of trimmed pubes, since he or she is plenty stocked up on dental floss at the moment, thankyouverymuch. Eyebrows never seem to grow more than a centimeter, if that, though recently I found two rogue ones in the left brow that were an inch—an entire inch!—long. I need to go in with scissors more often, I guess.

With all the grooming and manscaping that we do, I got curious about body hair—from head to toe–and why it grows differently—and stops growing at different points—from one body part to another. Why can the hair on my head grow past my butt, but by butt hairs will never sweep the floor? (Thankfully.) To learn more, I reached out to Dr. Baiju Gohil, MD, of Robotic Hair Restoration in Long Island. He’s a board-certified surgeon and Fellow of the American College of Surgeons, dedicated to hair restoration and hair loss treatment. Here’s what Gohil had to say about hair growth, and the rules it usually follows.

Why does hair grow at different lengths from one body part to the next?

Gohil says that the first thing to know about hair growth—anywhere on the body—is that it goes through a life cycle consisting of three different phases. This starts with the Anagen Phase, which “is the actual growth stage of the hair,” Gohil says. “For hair on the head, this phase lasts between 3-7 years for each hair follicle. However, for hair on our body this phase only lasts 3-6 months.” The stopping point, which is the maximum a hair can grow, is called the hair’s “terminal length”. Then there’s the Catagen Phase. “This is a short transitional phase after Anagen, where hair growth slows down until it stops. This phase takes between 10 days and three months.” Finally the Telogen Phase “is the resting phase where the hair stops growing, is released, and falls out,” says Gohil. “The follicle then goes into an inactive state for approximately three months, after which a new hair begins to grow and the entire process is repeated.”

Body hairs—like those on the chest, eyebrows, face, and other extremities—do not grow as long as the hairs on the head, obviously. “The longer the length of time in the Anagen Phase, the longer a hair can grow,” he says. “Each hair follicle independently goes through the growth cycle at different times, otherwise all your hair would fall out simultaneously. Instead, you only shed a certain number of hairs per day, around 80 hairs, from a healthy scalp.”

But… why?

“Anthropologists and biologists are still not sure exactly why, but many believe that the hair on our head grows longer due to practical evolutionary benefits,” Gohil says. “A lot of body heat escapes from the head, which houses the brain. Hair is a good insulator that can keep heat in, providing a survival advantage, especially in colder climates. Because man evolved into a bipedal being, the head became more directly exposed to the sun and required more protection. Therefore, another reason for the existence of scalp hair may be to provide protection against UV rays, and other potentially harmful environmental factors.”

How does the follicle “know” the length of its hair? If you trim a hair, does the follicle still grow a certain amount of time, as if we hadn’t trimmed it?

“A hair follicle does not ‘know’ its own length,” says Gohil. “All hair follicles go through the three growth phases mentioned above, the way they are pre-programmed to, despite trimming. Trimming has no influence on the growth cycle of a hair follicle. The hair follicle grows the same way, whether or not we cut the hair.”

How does hair length vary from one person to the next?

“The length of the hair within the growth cycle is not the same in everyone,” he says. “Many factors determine length, thickness, and the amount of hair on the head and body, genetics being just one of them.”

Why do some hairs—eyebrows come to mind—occasionally grow longer than the preset terminal length?

“There are a few different reasons why eyebrow hairs may violate the terminal length rule,” Gohil says. “As we age, skin sags and loses its fullness, making the eyebrow hairs appear longer, even though they are the same length. The keratin within each hair is not arranged as precisely as we get older, allowing keratin fibers to bend more easily than they used to. These more flexible hairs are not aligned with the others anymore in their usual eyebrow formation, and become more visibly apparent. In advanced age, hairs are also more hormone sensitive, especially to testosterone. After menopause, estrogen levels in women decrease, and the influence of naturally occurring testosterone in females is not counteracted, which may lead to more pronounced eyebrow hair.”